1. Field of the Invention
The present invention relates to an image-bearing member protecting agent that is applied or adhered to a surface of an image bearing member so as to protect the surface thereof, a protective layer forming device for applying or adhering the protecting agent to the surface of the image bearing member so as to form a protective layer thereon, an image forming method, an image forming apparatus and a process cartridge, using the image-bearing member protecting agent or the protective layer forming device, in an electrophotographic image forming method and image forming apparatus.
2. Description of the Related Art
Conventionally, in electrophotographic image formation, a latent electrostatic image is formed on an image bearing member made from a photoconductive material, and charged toner particles are attached to this latent electrostatic image so as to form a visible image. The visible image formed with the toner particles is transferred onto a recording medium such as paper, or the like, and then fixed on the recording medium utilizing heat, pressure, solvent gas, or the like so as to form an output image.
Methods for the image formation are broadly classified, according to methods for charging toner particles to form a visible image, into so-called two-component developing methods in which frictional charging effected by stirring and mixing toner particles and carrier particles is utilized, and so-called one-component developing methods in which toner particles are charged without using carrier particles. Further, the one-component developing methods are classified into magnetic one-component developing methods and nonmagnetic one-component developing methods, according to whether or not magnetic force is utilized to keep toner particles on a developing roller.
In image forming apparatuses, such as copiers, complex machines based upon the copiers, and the like for which high-speed processing capability and image reproducibility are required, the two-component developing methods have been employed in many cases due to demands for stable chargeability of toner particles, stable charge rising properties of the toner particles, long-term stability of image quality, and the like; whereas in compact printers, facsimiles, etc. for which space saving, cost reduction and the like are required, the one-component developing methods have been employed in many cases.
Also, nowadays in particular, colorization of output images is progressing, and demands for improvement of image quality and stabilization of image quality are increasing like never before.
For the improvement of image quality, toners have been made smaller in average particle diameter, and particles of the toners have been made rounder in shape with their angular parts removed.
Generally, in an image forming apparatus which operates in accordance with any such electrophotographic image forming method, regardless of which developing method is employed, a drum-shaped or belt-shaped image bearing member (typified by a photoconductor) is uniformly charged while being rotated, a latent image pattern is formed on the image bearing member by laser light or the like, and the latent image pattern is visualized as a toner image by a developing unit and transferred onto a transfer medium.
After the toner image has been transferred onto the transfer medium, untransferred toner components remain on the image bearing member. If such residues are directly conveyed to a region for the charging step, it often hinders the image bearing member from being uniformly charged; accordingly, in general, the toner components, etc. remaining on the image bearing member are removed by a cleaning unit in a cleaning step after the transfer step, thereby bringing the surface of the image bearing member into a clean enough state, and then charging is carried out.
Thus, the image bearing member surface is exposed to various types of physical stress and electrical stress in each steps of charging, developing, transferring and cleaning, and the like, and a state of the image bearing member surface changes over time.
Of these stresses, it is known that the stress caused by friction in the cleaning step wears the image bearing member, and generates scratches. In attempts to solve this problem, a number of proposals for lubricants and methods of supplying lubricant components and forming films have been made thus far to reduce frictional force between the image bearing member and cleaning member.
For example, Japanese Patent Application Publication (JP-B) No. 51-22380 proposes a method of forming a lubricant film on a photoconductor surface by supplying the photoconductor surface with a solid lubricant composed mainly of zinc stearate in order to lengthen the lifetimes of a photoconductor and a cleaning blade.
JP-A No. 2005-274737 discloses that a lubricant supply device for supplying a lubricant mainly containing a higher alcohol having 20 to 70 carbon atoms is used, so that the higher alcohol remains in a form of irregular shaped particles on an edge of a blade nip portion, and has suitable wettability to an image bearing member surface, thereby exhibiting continuously lubricating performance.
JP-A No. 2002-97483 discloses that a powder of a certain alkylene-bis-alkyl acid amide compound as a lubrication component is used so as to provide the powder fine particles at the interface where an image bearing member and a cleaning blade are in contact with each other, thereby maintaining smooth lubrication effect on the surface thereof for a long period.
JP-A No. 2005-171107 discloses that various lubricants obtained by adding an inorganic lubricant to a solid lubricant containing zinc stearate as a main component are supplied to a photoconductor (image bearing member) surface, so as to decrease a frictional force between the image bearing member and a cleaning member.
JP-A No. 2006-350240 discloses that various lubricants obtained by adding boron nitride to a solid lubricant containing zinc stearate as a main component are supplied to a photoconductor (image bearing member) surface, so that lubricity is not easily decreased even when the image bearing member surface is subjected to electrical stress in a charging step, and the lubricant is formed into a film all over the image bearing member surface, thereby maintaining high lubricity.
On the other hand, nowadays, a toner produced by a polymerization method is commercially available in order to enhance image quality and reduce production energy. The polymerized toner has excellent characteristics, for example, it has less angular shape and a small and uniform average particle diameter, compared to a toner produced by a pulverization method. However, in a system in which an edge portion of a cleaning member such as a rubber cleaning blade is pressed to be in contact with an image bearing member surface so as to clean the image bearing member surface, the toner is hard to be blocked at the edge portion, and cleaning failure of the residual toner component easily occurs, due to the shape and particle diameter of the toner.
Some techniques dealing with such toner cleaning failure have been proposed.
For example, according to the technique disclosed in JP-A No. 2007-286594, the use of a hydrophobic and amphipathic organic component allows to remarkably improve cleanability of a toner. Thus, a toner of recent years, which has a small particle diameter and high sphericity, can be cleaned. Moreover, smearing on a charging member is decreased due to improvement in cleanability, and the charging member achieves a longer operating life. Furthermore, since the toner does not pass through a blade, the blade is less worn, and the cleaning blade also achieves a longer operating life.
However, stress on the image bearing member is caused not only in the cleaning step, as described above. Particularly, an electrical stress in the charging step significantly changes a state of a surface of the image bearing member. Moreover, the electrical stress outstandingly occurs in a contact charging system or a close contact charging system, in which a discharging phenomenon occurs near the surface of the image bearing member. In these charging systems, many active species and reaction products are generated on the surface of the image bearing member, and a large amount of the active species and reaction products generated in an atmosphere of a discharge region are adsorbed on the surface of the image bearing member.
A lubricant using zinc stearate, for example, similar to the conventional technique disclosed in JP-B No. 51-22380, relatively uniformly covers the surface of the image bearing member to provide excellent lubricity and protective properties. Thus, zinc stearate is used to prevent the photoconductor wear which is a problem caused in an image forming process, in which AC voltage is applied to charge the image bearing member.
However, zinc stearate has a problem in cleanability. In a normal image forming process, a blade cleaning system is used to remove a residual toner on a photoconductor, from which a toner image has been transferred. However, zinc stearate has properties to make toner more likely to pass through the blade. When the toner passes through the cleaning blade, the toner is directly printed as an image, and smearing on the charging member is further increased. The more significantly the toner passing-through the blade is exhibited, the more spherical and the smaller particle diameter the toner has. Meanwhile, since the lubricant using zinc stearate causes a large amount of the toner passing through the blade, the cleaning blade is worn, shortening the operation life of an image forming apparatus.
In the cleaning step, a large amount of zinc stearate as well as the toner pass through the blade, causing smearing on the charging member. Particularly, in a system in which a charging roller is in contact with or closely adjacent to a photoconductor so as to charge the photoconductor, the smearing on the charging member is increased. When the charging member is smeared, an abnormal image having uneven density is formed due to uneven charge.
In the conventional technique as described in JP-A No. 2005-274737, the lubricant containing higher alcohol easily makes an image bearing member surface wet, and it is expected to exhibit an effect as the lubricant. However, an adsorption area per molecule of the higher alcohol molecule adsorbed on the image bearing member is likely to be broad, and a density of a molecule adsorbed on the image bearing member per unit area (a weight of an adsorbed molecule per unit area of the image bearing member) is small. Thus, the electrical stress easily goes through the protective layer, and it is hard to achieve an effect for sufficiently protecting the image bearing member by using the lubricant.
According to the conventional technique described in JP-A No. 2002-97483, in the case where the lubricant contains a nitrogen atom in a molecule, when the lubricant itself is exposed to the above-mentioned electrical stress, the lubricant in a molecule produces an ionic dissociating compound as a decomposed product like a nitrogen oxide and an ammonium-containing compound, and the ionic dissociating compound is taken into the lubricant layer. Then, the resistance of the lubricant layer is decreased at high humidity, and image blur may occur.
JP-A No. 2005-171107 discloses that fine particles, such as of silica, titania, alumina, magnesia, zirconia, ferrite, and magnetite, are added in a solid lubricant containing zinc stearate as a main component used on a photoconductor surface. However, the toner passing through the blade is not significantly improved by using these inorganic fine particles, and the smearing on the charging roller is not decreased. As a result, an entire image forming apparatus has a short operation life. Moreover, inorganic fine particles leave scratches or scars on the image bearing member, which may cause formation of an abnormal image.
The lubricant disclosed in JP-A No. 2006-350240 has protective properties from the charging member, and remarkably prevents toner from passing through the blade, thereby preventing the charging member from smearing. However, the protecting agent deposits on the image bearing member, causing filming thereon.
The protecting agent disclosed in JP-A No. 2007-286594 does not have sufficient protective properties from charging, causing severe abrasion of the photoconductor. In this case the abrasion speed of the photoconductor becomes 10 times or more faster than that when the zinc stearate is used as described in JP-B No. 51-22380. The abrasion can be decreased to some extend by increasing the amount of the protecting agent coated onto the photoconductor. However, it is impossible to prevent the abrasion completely. Moreover, it is understood that in the case where the amount of the protecting agent is increased, the lubricant is adhered on the photoconductor, causing formation of an abnormal image.